Piezoelectric materials comprising ferroelectric ceramics have heretofore been used in various applications such as piezoelectric filters, piezoelectric transducers, ultrasonic oscillators or piezoelectric buzzers. Typical ferroelectric ceramics used in such applications are solid solutions of PbTiO.sub.3 -PbZrO.sub.3 series. Further, solid solutions of PbTiO.sub.3 -PbZrO.sub.3 series combined with a solid solution of a complex Perovskite type compound such as Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3, Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 or Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 are also known as ferroelectric ceramics having improved piezoelectric properties.
On the other hand, the use of piezoelectric ceramic materials as an actuator has recently been studied. In this case, it is necessary to transform electric energy to mechanical energy by the piezoelectric ceramic material. In order to transform the energy piezoelectric ceramic materials having a large piezoelectric constant d are desired.
Generally, the piezoelectric constant d of a piezoelectric ceramic material is related to an electromechanical coupling factor k and a relative dielectric constant .epsilon. of the piezoelectric ceramic material by the equation ##EQU1## Therefore, in order to increase the piezoelectric constant d, the electromechanical coupling factor k and/or the relative dielectric constant .epsilon. must be increased.
Further, in applications of piezoelectric ceramic materials as a driving part of an actuator, for such as in an ultrasonic motor, where mechanical resonance of the material is utilized, it is desired that both the piezoelectric constant d and the mechanical quality factor Qm are large. That is, if a piezoelectric ceramic material is used in an ultrasonic motor, i.e. in the resonant frequency region, and the mechanical quality factor Qm of the material is small, then the amplitude of the mechanical resonance of the material will not become large and the heat generated by the high frequency driving involved will become large. This leads to an undesirable reduction in the spontaneous polarization of the material and to an undesirable decrease in piezoelectric constant d of the material.
Similarly, it is desired that when piezoelectric ceramic materials are used as an actuator, they posses a high Curie temperature, Tc, so that the in spontaneous polarization and the piezoelectric constant d of the material are not undesirably reduced by the high heat generated by the high frequency driving of the material further a high TC allows for a broader temperature range at which the material is operable as the actuator.
While the mechanical quality factor Qm of solid solutions of PbTiO.sub.3 -PbZrO.sub.3 series or of PbTiO.sub.3 -PbZrO.sub.3 series combined with a solid solution of a complex Perovskite, such as Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3, Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 or pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 can be improved by incorporating a hard compound, such as MnO.sub.2 therein. But, there is been a problem with such modification in that the piezoelectric constant d of the material is drastically decreased as the amount of MnO.sub.2 incorporated increases.
While the piezoelectric constant d of the above-mentioned solid solutions can be improved, on the one hand, by the incorporation of a soft compound such as Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, La.sub.2 O.sub.3, Nd.sub.2 O.sub.3 or Bi.sub.2 O.sub.3 therein; or by substituting parts of Pb of the above-mentioned solid solutions with Ba, Sr or Ca, there is a problem with such a modification in that the Curie temperature of the material is drastically decreased as the amount of the soft compound or of the substitution amount increases.
In light of the problems as mentioned above, the inventors conducted thorough investigations in metal ion substituents that may be used in Perovskite solid solutions of PbTiO.sub.3 -PbZrO.sub.3 series, and as the result it has been found that modifying solid solutions of Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 -Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 -PbTiO.sub.3 -PbZrO.sub.3 series by substituting parts of Pb of the solid solutions with La or Nd in a specific amount and incorporating a specific amount of MnO.sub.2 produces a ferroelectric ceramic that exhibits a more stable piezoelectric constant d, an improved mechanical quality factor Qm and, a high Curie temperature Tc. The present invention has been accomplished on the basis of the above findings.